The small intestine is made up of the duodenum, jejunum, and ileum. Together with the large intestine and the stomach it forms the gastrointestinal tract. In living humans, the small intestine alone measures around six meters. After death, this length can increase by up to half. It has a surface area of over 200 meters The internal walls of the organ are covered in villi, themselves harbouring subsections called microvilli, which increase the surface area available for the absorption of nutrients. Food already broken down by chewing and stomach enzymes, is degraded by further enzymes. Some of these chemicals are secreted in the lumen, but others are transported to the intestine from other organs such as the pancreas and liver. Depending on the type of nutrient or vitamin being absorbed, this may take place in different sections of the small intestine. Once fully reduced to a chemical level the molecules pass through the walls of the intestine into the bloodstream. Peristalsis, contraction of the muscle walls, is the force that propels matter through the small intestine. It is a slow process, allowing the food matter to mix with the digestive juices.